As an access network involves a large quantity of end users and complexly distributed optical link branches, and an all-fiber passive network is used for an entire network, how to implement appropriate monitoring and maintenance on optical links becomes a technical problem that needs to be urgently resolved by an operator. Currently, a monitoring system of an operator mainly uses an optical time domain reflectometer (OTDR) to test a link. A relatively mainstream manner of the testing is: using a 1×N optical switch and a dense wavelength division multiplexer (DWDM) to access N access ends. Specifically, FIG. 1 is a schematic diagram of a principle of a monitoring system in the prior art. As shown in FIG. 1, the monitoring system may be placed in a central office optical distribution frame (ODF), and monitors multiple passive optical networks (PONs), where the PONs include an optical line terminal (OLT), and the monitoring system includes an OTDR 11, a 1×N optical switch 12, and N WDMs 13. An operating principle of the monitoring system is as follows: The OTDR 11 is connected to an input end of the 1×N optical switch 12; after the OTDR 11 inputs OTDR test light to the 1×N optical switch 12 by using the input end of the 1×N optical switch 12, the OTDR test light may be switched to a particular output channel by using a control system; then, by using a WDM 13 corresponding to the particular output channel, the OTDR test light is coupled to a corresponding PON, and is transmitted downstream to an ONU to generate backscattering, where backscattered light coupled to the PON is transmitted upstream to the WDM 13 and is returned to the OTDR 11.
However, each output end of a 1×N optical switch needs one WDM, and therefore, when a large-split ratio optical switch is used, costs of the monitoring system and difficulty in managing a fiber patch cord between the optical switch and a WDM may be increased. In addition, space pressure on an equipment room whose space resource is already tight is also increased.